[PATCH v2] t/perf worktree improvements

I don't think v1 of this patch got picked up at all, so here it is again. There was a question of whether we could do the much simpler solution discussed in:

  https://lore.kernel.org/git/22378ce3-6845-1cd9-996a-8bdc3a8b65d7@gmail.com/

But I think it would be confusing. So patch 1 is unchanged here from v1.

Johannes suggested we could add some extra protections to avoid accidentally modifying the original repo. Patch 2 does that.

  [1/2]: t/perf: handle worktrees as test repos
  [2/2]: t/perf: avoid copying worktree files from test repo

 t/perf/perf-lib.sh | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

-Peff